Industrial and utility bearings are produced in many sizes and configurations, but for the most part have some common structural elements related to the bearing backing and the soft metal or babbitt materials deposited thereon. Such bearings withstand difficult duty during operation. To understand what type of considerations are taken into account in the design of industrial bearings, it is helpful to look at the kind of operating conditions bearings can be subjected to. For example, a journal bearing for a 750 megawatt large steam turbine-generator low pressure rotor. Typically, such a rotor will have two bearings per span, together supporting upwards of 95 tons rotating at 1800 RPM and experiencing vibration of up to 2 mils peak-peak during regular operation. Due to a variety of circumstances, the vibration levels may exceed 2 mils under alarm conditions, with the unit set to trip off line at around 5 mils to avoid damage to the unit. At 5 mils peak-peak vibration, a journal bearing of 36 inch inside diameter takes a tremendous beating, yet must substantially maintain its surface and shape. Bearing failure at such extreme conditions may be disastrous, resulting in damage to the bearing journals requiring journal polishing or grinding costing several thousands of dollars and lost revenue, or even complete destruction of stationary pieces such as packing and seals, or even the rotor itself, requiring repairs in the millions of dollars.
Thrust bearings see a different duty, carrying the axial thrust of such rotors wanting to travel downstream with the steam flow, and in the event of failure, into stationary elements destroying them or even the rotor itself. Bearings near the generator of such a unit are even subjected to residual eddy currents sometimes causing etching of the metallic surface of the bearing. Bearings are particularly susceptible to conditions of lube oil systems such as oil temperature, flow or cleanliness, alignment of rotating components, and assembly of the bearing housing itself.
Since bearings are so critical to the proper operation of a variety of equipment, much attention has been paid to the design of bearings, bearing housings, lubrication systems and bearing metal surface configuration. Bearings can be found with elliptical or circular inside diameters, tilting pads, holes for jets of oil from lift pumps to allow extremely heavy rotors to begin rotation on an oil film, and self-aligning spherical seated rings to allow the bearing to react to the expansion, contraction and elevational changes of the rotor it supports.
Regardless of the varying configurations just mentioned, representing a small fraction of those in use, or the extreme duty described above, a common denominator exists with nearly all babbitted bearings: the critical importance of the integrity of the bond between the soft bearing babbitt or "white metal" and the bearing backing or shell to which it is attached. Because bearings see such extreme duty, degrading over time due to oil conditions or even the opening up of bearing clearance from wear or pounding, the composition of the babbitt and, more importantly, the integrity of the bond are critical to the reliable operation of the bearing.
Babbitting is a process named after Isaac Babbitt who patented a process in the United States in 1863, for bonding soft metals to a stronger shell or stiffener used to support the weight and torsion of a rotating, oscillating or sliding shaft. The soft metal prevents galling or scoring of the shaft for long periods. Babbitt materials are generally comprised of tin alloys or lead alloys, each usually combined with copper for ductility and antimony for hardness, and arsenic for hardness in the case of lead alloys. The base metal may be in the form of mild steel strip unwound from a coil, a half-round mild steel pressing or bushing, or castings or forgings of iron, steel or bronze. The bonded bimetal material is formed and machined to make plain, fluid film lubricated bearings for a wide variety of automotive, industrial and marine applications.
Babbitting of bearing backings and shells can be accomplished typically by either statically casting, centrifugally casting or tig welding the babbitt onto the backing. Centrifugal ("spin") casting of journal bearings offers both technical and often economic advantages. Thrust bearings, usually flat but sometimes in the form of "tapered land" bearings, are generally statically cast. The pads from tilting pad journal bearings are also generally centrifugally cast. In the case of pad-type bearings, the pad may have a base material consisting of steel, iron, bronze, copper or copper-chromium alloys such as that specified for use by the Westinghouse Electric Corporation under the trade name "cupalloy," or as specified by the common copper alloy material specification CDAC 18200, or its equivalent.
Regardless of the method of depositing babbitt onto the backing, the quality of the babbitt to backing bond is particularly important. A metallurgical or chemical bond is required to ensure good heat transfer from the babbitt into the backing and provide satisfactory babbitt fatigue life. Dovetails and tapped holes are commonly used in cast iron shells to provide a mechanical bonding of the babbitt metal, and are commonly used in concert with chemical bonding to further ensure retention of the babbitt on the backing.
Prior to casting, the workpiece is meticulously prepared by various cleaning, fluxing and tinning steps. A good description of the preparation steps for babbitting such bearings can be found in "Babbitting", Volume 5, Surface Engineering, in the trademarked ASM HANDBOOK.RTM., ISBN 0-87170-384-X, by William P. Bardet and co-inventor of the instant invention, Donald J. Wengler.
Environmental concerns and constraints have caused industry to avoid the use of lead alloys for babbitt in favor of tin alloys. In the case of copper or copper-chromium alloy tilt pads, a problem arises between the pad, acting as the bearing backing in this case, and the babbitt material. If a copper or copper-chromium pad is tinned and babbitted, there occurs a migration of tin into copper forming an intermetallic layer of typically Cu.sub.6 Sn.sub.5 or Cu.sub.5 Sn.sub.4. When molten tin is applied directly to steel backings, the tin layer will not grow after solidification, but in the copper alloy pad, the resulting intermetallic layer will continue to grow even after solidification. This growth is further stimulated by elevated temperature, and growth may continue until the intermetallic layer ultimately fractures due to its brittle nature. Fracturing of the intermetallic layer results in a lack of bond between the babbitt and the backing. Most original equipment manufacturers (OEM's) and the United States Department of Defense require that there be a minimum of a 90% and as much as 100% bonding (given the specific area of bond evaluated) of the babbitt to the backing as determined by non-destructive testing methods, such as ultrasonic examination. Repairing a discrete area of lack of bond or babbitt surface damage is sometimes done by "puddling" an amount of molten babbitt into a prepared area of the backing, and then resurfacing the puddled babbitt. However, many OEM's require repairs to include a completely new casting of babbitt because of the localized heating of the puddled babbitt and its inherent metallurgical difference from the original babbitt material, which has changed physically due to operational influences.
While various layer material compositions have been employed to minimize this copper-tin migration problem, none has succeeded completely. Nickel has been used by bearing manufacturers to minimize the problem with some success, but the migration of copper and tin toward each other still occurs, even if at a lower rate, due to the similar migration characteristics of tin and nickel. What is needed then is a metallic barrier layer which will provide an electro-chemical and/or mechanical bond between the babbitt and the copper or copper-chromium alloy, while eliminating the migration of copper and tin to form Cu.sub.6 Sn.sub.5 or Cu.sub.5 Sn.sub.4. To date, the prior art has not taught such a barrier layer.
While plating the bearing backing with a nickel alloy is currently a common method of depositing a barrier layer material, nickel requires a plating process that is expensive and of higher environmental impact due to the toxicity of the process byproducts and the cost to mitigate potential dangers and dispose of hazardous waste. What is needed then is a metallic barrier material which is less toxic and which employs a plating process which has a reduced impact on the environment.
Iron may be deposited from a variety of electrolytes and is also a quite inexpensive metal. A good discussion of iron and iron plating processes can be found in the article "Iron Plating", by Sue Troup-Packman, co-inventor of the instant invention, Volume 5, ASM HANDBOOK.RTM., cited above. Many of the electrolytes used in iron plating are very corrosive however. Common iron plating methods are typically too fast and aggressive for such an application as bearing bonding layers, depositing too much material of a lower density. Because of the good bonding properties of iron relative to copper-chromium or copper and tin alloys, a process of depositing iron via a less toxic process onto a backing and thereby enabling a bond resistant to the formation of Cu.sub.6 Sn.sub.5 or Cu.sub.5 Sn.sub.4, would constitute a novel and advantageous approach to bearing manufacture and repair.